This invention relates to an optical disc and an optical disc reproducing device which uses this optical disc, and particularly to an optical disc which has a plurality of recording layers and an optical disc reproducing device which uses this optical disc.
Conventionally, as a recording medium for various types of information such as audio information and video information, there has been broadly used an optical disc such that information recorded thereon is reproduced using a laser beam. For this type of optical disc, an optical disc having multiple recording layers has been proposed in order to realize further increase in the quantity of information that can be recorded.
As an optical disc having multiple recording layers, there has been proposed an optical disc as described in the Japanese Publication of Unexamined Patent Application No. Hei 8-235641.
In the optical disc 1 described in this publication, a first recording carrier 4a constituted by forming a first recording layer 3a on one side of an optically transparent first substrate 2a and a second recording carrier 4b constituted by forming a second recording layer 3b on one side of an optically transparent second substrate 2b are arranged in such a manner that the sides where the first and second recording layers 3a and 3b are formned face each other, and these first and second recording carriers 4a and 4b are joined with each other via an optically transparent adhesive layer 5, as shown in FIG. 1.
On the side where the first and second recording layers 3a and 3b are formed, of the first and second substrates 2a and 2b constituting the first and second recording carriers 4a and 4b, information such as video information is recorded in the form of pit patterns 6a and 6b made of minute recesses and protrusions. On the pit pattern 6a provided on the first recording carrier 4a, the first recording layer 3a is formed by depositing a semitransparent film of SiO, SiO2 or the like along the pit pattern 6a by evaporation or sputtering. The first recording layer 3a has a reflectance of 20 to 50% and an optical transmittance of 30 to 80%. On the pit pattern 6b provided on the second recording carrier 4b, the second recording layer 3b is formed by depositing an aluminum evaporation film along the pit pattern 6b. The second recording layer 3b has a reflectance of 60% or higher, but the reflectance of a laser beam transmitted through the first recording layer 3a and reflected by the second recording layer 3b is 20 to 50%.
With such an optical disc 1 having the first recording layer 3a which is optically semitransparent, information recorded on the second recording layer 3b can be reproduced by casting a laser beam L2 from the same direction as the direction of casting a laser beam L1 to the first recording layer 3a, as shown in FIG. 1. In this case, selection of either the information recorded on the first recording layer 3a or the information recorded on the second recording layer 3b is carried out by changing the focal positions of the laser beams L1 and L2 cast onto the optical disc 1.
As another optical disc having multiple recording layers, an optical disc as shown in FIG. 2 has been proposed. In this optical disc 11, a first recording carrier 14a constituted by forming a first recording layer 13a on one side of an optically transparent first substrate 12a and a second recording carrier 14b constituted by forming a second recording layer 13b on one side of an optically transparent second substrate 12b are joined with an adhesive layer 15 made of an optically transparent ultraviolet-cured resin or the like, as shown in FIG. 2. The first and second recording layers 14a and 14b are joined in parallel in such a manner that the second substrate 12b is located on the first recording layer 13a. On the second recording layer 13b, a protection layer 17 for protecting the second recording layer 13b is provided.
On the sides where the first and second recording layers 13a and 13b are formed, of the first and second substrates 12a and 12b constituting the first and second recording carriers 14a and 14b, information such as audio information is recorded in the form of pit patterns 16a and 16b made of minute recesses and protrusions. On the pit pattern 16a provided on the first recording carrier 14a, the first recording layer 13a having wavelength selectivity is formed along the pit pattern 16a. The first recording layer 13a is formed, for example, as a five-layer film made up of Si3N4, SiO2, Si3N4, SiO2, and Si3N4. The first recording layer 13a has a reflectance of approximately 34% for a laser bean with a wavelength of 635 nm, and has a reflectance of approximately zero for a laser beam with a wavelength of 780 nm and thus substantially transmits this laser beam.
On the pit pattern 16b provided on the second recording carrier 14b, the second recording layer 13b is formed by depositing an aluminum evaporation film with a high reflectance along the pit pattern 16b. The second recording layer 13b has a reflectance of 80% or higher for the laser beam transmitted through the first recording layer 13a. The second recording layer 13b has a reflectance of 84% or higher for the laser beam with a wavelength 780 nm, which is substantially transmitted through the first recording layer 13a, and has a reflectance of approximately 38% for the laser beam with a wavelength of 635 nm, which is reflected approximately 34% through the first recording layer 13a. 
In the optical disc 11 thus constituted, information recorded on the first and second recording layers 13a and 13b is reproduced by casting a laser beam from the side of the substrate 12a of the first recording carrier 14a. In this case, if a laser beam L3 with a wavelength of 780 nm is used as a laser beam for reading the information, this laser beam is transmitted through the first recording layer 13a and cast onto the second recording layer 13b, and a return laser beam reflected from the second recording layer 13b is obtained. By detecting this return laser beam, the information such as audio information recorded on the second recording layer 13b is read and reproduced. The laser beam L3 with a wavelength of 780 nm is what is used for reproduction of an optical disc using a reproduction-type optical disc with a diameter of 12 cm, that is, a so-called compact disc, as a recording medium. Therefore, the information recorded on the second recording layer 13b can also be reproduced by a general-purpose optical disc reproducing device.
If a laser beam L4 with a wavelength of 635 nm is cast from the side of the substrate 12a of the first recording carrier 14a, a return laser beam reflected from the first recording layer 13a and the second recording layer 13b can be obtained. Specifically, since the first recording layer 13a has a reflectance of approximately 34% for the laser beam with a wavelength of 635 nm, this laser beam is transmitted through the first recording layer 13a and becomes incident on the second recording layer 13b, and a return laser beam reflected from the second recording layer 13b is obtained. The reflectance for the laser from the second recording layer 13b in this case is approximately 34%.
In this optical disc 11, by using the laser beam L4 with a wavelength of 635 nm and setting the focal position of the laser beam L4 on the first or second recording layer 13a or 13b, the information recorded on the first or second recording layer 13a or 13b can be reproduced.
In the optical disc 1 shown in FIG. 1 in which the first and second recording carriers 4a and 4b are joined in such a manner that the sides having the first and second recording layers 3a and 3b formed thereon face each other, reproduction of the information recorded on the first or second recording layer 3a or 3b can be selectively carried out by setting the focal position of the laser beam cast onto the optical disc 1 to the first or second recording layer 3a or 3b, as described above. In the optical disc 1, since the first and second recording layers 3a and 3b are arranged closely to each other, a detection error tends to occur in the case of detecting the focal position of the laser beam and detecting the first or second recording layer 3a or 3b. Therefore, it is difficult to securely focus the laser beam to a desired one of the first and second recording layers 3a and 3b, and to accurately reproduce desired information.
It is conceivable to detect the difference in reflectance between the first and second recording layers 3a and 3b and thus detect which of the first and second recording layers 3a and 3b is the layer on which the laser beam cast on the optical disc 1 is focused, thus selecting the first or second recording layer 3a or 3b. However, in this optical disc 1, since the first and second recording layers 3a and 3b have substantially the same reflectance for the laser beam cast on the optical disc 1, the difference in reflectance for the laser beam cannot be used for selecting the first or second recording layer 3a or 3b. 
Meanwhile, in the optical disc 11 shown in FIG. 2 which has the first recording layer 13a having wavelength selectivity, the information recorded on the first recording layer 13a can be reproduced by a general-purpose optical disc reproducing device which uses the above-described compact disc as a recording medium, but a dedicated optical disc reproducing device which uses a laser beam with a wavelength of 635 nm is required for reproducing the information recorded on the first and second recording layers 13a and 13b. In the optical disc 11, too, the first and second recording layers 13a and 13b have substantially the same reflectance for the laser beam cast onto the optical disc 11, land therefore detection of the difference in reflectance for the laser beam cannot be used for selecting the first or second recording layer 13a or 13b. 
As described above, though the conventionally proposed optical discs realize an increase in the quantity of recording information by providing multiple recording layers, it is difficult to select each recording layer and accurately focus a laser beam onto the selected recording layer. Therefore, it is not possible to easily and accurately reproduce desired information.
Thus, in view of the foregoing status of the art, it is an object of the present invention to provide an optical disc and an optical disc reproducing device which enable realization of multiple recording layers for increasing the recording capacity and accurate reproduction of information recorded in each recording layer.
It is another object of the present invention to provide an optical disc and an optical disc reproducing device which enable easy and accurate selection of a desired signal recording layer and accurate reproduction of desired information, using the difference in reflectance.
It is still another object of the present invention to provide an optical disc and an optical disc reproducing device which enable selective reproduction of a plurality of signal recording layers using a single laser beam.
It is still another object of the present invention to provide a highly general-purpose optical disc which enables reproduction by an optical disc reproducing device using a conventionally used compact disc as a recording medium.
It is still another object of the present invention to provide an optical disc which can be easily manufactured while realizing increase in the quantity of recording information.
It is a further object of the present invention to provide an optical disc reproducing device of a simple structure which enables simplification of the structure of an optical pickup used for reproducing information.
An optical disc according to the present invention comprises: at least a first recording layer and a second recording layer provided as being stacked on one side of a substrate; and a protection layer for protecting the recording layer located farther from the substrate, of the first recording layer and the second recording layer; wherein the reflectance of one of the first recording layer and the second recording layer is made higher than the reflectance of the other recording layer.
In this optical disc, the reflectance of the recording layer located farther from the substrate, of the first recording layer and the second recording layer, is made higher than the reflectance of the recording layer located closer to the substrate, of the first recording layer and the second recording layer.
The optical disc according to the present invention further comprises an optically transparent layer provided between the first recording layer and the second recording layer. This optically transparent layer can be used for joining the first recording layer with the second recording layer.
Another optical disc according to the present invention comprises: a first recording layer provided on one side of a substrate; a second recording layer provided in parallel to the first recording layer; and a protection layer for protecting the recording layer located farther from the substrate, of the first recording layer and the second recording layer; wherein the reflectance of one of the first recording layer and the second recording layer is made higher than the reflectance of the other recording layer.
A reproducing device according to the present invention uses, as a recording medium, an optical disc comprising at least a first recording layer and a second recording layer provided as being stacked on one side of a substrate, and a protection layer for protecting the recording layer located farther from the substrate, of the first recording layer and the second recording layer, wherein the reflectance of one of the first recording layer and the second recording layer is made higher than the reflectance of the other recording layer. The reproducing device comprises: a single light source for emitting a light beam to be cast onto the optical disc; and a single objective lens for condensing the laser beam emitted from the light source to the first recording layer or the second recording layer via the substrate.
The reproducing device further comprises: a photodetector for receiving a return light from the first recording layer or the second recording layer; a driving section for driving the objective lens at least in a direction parallel to the optical axis of the objective lens; and a control section for controlling the driving section to condense the light beam emitted from the light source onto either the first recording layer or the second recording layer.
An optical disc reproducing method according to the present invention comprises the steps of: casing a light beam emitted from a single light source to an optical disc via a single objective lens, the optical disc comprising at least a first recording layer and a second recording layer provided as being stacked on one side of a substrate, and a protection layer for protecting the recording layer located farther from the substrate, of the first recording layer and the second recording layer, wherein the reflectance of one of the first recording layer and the second recording layer is made higher than the reflectance of the other recording layer; discriminating which of the first recording layer and the second recording layer the light beam emitted from the light source is cast on, on the basis of the result of detection of a reflected beam from the first recording layer or the second recording layer; and moving the objective lens in a direction parallel to the optical axis of the objective lens on the basis of the result of discrimination so as to condense the light beam emitted from the light source onto the first recording layer or the second recording layer, thus reproducing information recorded on the first or second recording layer.
The other objects and advantages of the present invention will be clarified further in the following description of embodiments.